1. Field of the Invention
The present invention relates generally to magnetic sensors, and more particularly, to magnetic sensors integrated with semiconductor devices.
2. Background of the Invention
Magnetoelectronics is a growing field that is devoted to the development of electronic device structures that incorporate a ferromagnetic element. As shown in FIG. 1, when a write current (Iw) is applied to an integrated, contiguous write wire 11 that is directly over a ferromagnetic element 12, a magnetic field (H) is generated that is parallel with and close to a surface of the write wire 11. The magnitude of the magnetic field (H) is determined by an inductive coefficient (α) and the write current (Iw), i.e., H=αIw. The magnetization of the ferromagnetic film is a function of the magnetic field and follows a hysteresis loop like that shown in FIG. 2.
The bi-stable orientation characteristic of the hysteresis loop of FIG. 2 is a defining characteristic of ferromagnetic materials and a natural basis for nonvolatile bit storage. Basically, when the magnetic field is larger than a switching field (Hs), the magnetization of the ferromagnetic film reaches a first saturation value (Ms). The magnetization is thereafter maintained at this first saturation value and particular orientation for periods as long as years, even when power is removed. The orientation of the magnetization changes when a magnetic field with a reversed direction is applied to the ferromagnetic element. The magnetization, however, drops down slightly when the reversed magnetic field is applied until the reversed magnetic field is less than −Hs. In this situation, the magnetization and output voltage jump promptly from the first saturation value (Ms, Vout) to the second saturation value (−Ms, −Vout), as shown in FIG. 2. The magnetization state is then maintained at the second saturation value for extremely long periods unless the magnetic field reaches Hs again.
Magnetoelectronic devices leverage the hysteresis-loop characteristic of ferromagnetic material to perform specific functions, such as “latching” data, Boolean operations and like functions. To detect a result of a Boolean operation, for example, magnetoelectronic devices also require a magnetic field sensor to detect the magnetic field induced by the ferromagnetic material.
Magnetic field sensors based on the Hall-effect are presently the most widely used magnetic sensor. When a magnetic field is applied perpendicularly to an electric conductor, a voltage is generated transversely to a current flow direction in the electric conductor. This phenomenon is called the Hall effect and the voltage generated is called Hall voltage. Therefore, magnetoelectronic devices typically utilize a Hall sensor to sense the orientation of the magnetic field induced by a magnetic element.
One example of a magnetoelectronic device is described in Mark Johnson et al.'s article entitled “Hybrid Hall Effect Device” which was first published in 1997. In Johnson et al.'s article, a single microstructured ferromagnetic film and a micro scale Hall cross are fabricated together to create a magnetoelectronic device. Magnetic fringe fields from the edge of the ferromagnet generate a Hall voltage in the Hall cross. The sign of the fringe field, as well as the sign of the output Hall voltage, is switched by reversing the magnetization of the ferromagnet. The Hall cross thus detects the Hall voltage and outputs a value (high or low) corresponding to the direction of the magnetization of the ferromagnet.
Hall sensors are not only used for detecting a magnetic field. Hall sensors also provide signals that can be used for implementing various sensing and control functions. Discrete Hall sensors, coupled with current-excitation and signal-conditioning blocks, provide a voltage output in the presence of a magnetic field. A number of integrated circuit sensor ICs ease the design task by combining Hall sensors and peripheral circuitry to provide linear or switched outputs. The majority of presently-available Hall sensors are low-cost discrete devices. The allure of contactless sensing, low parts cost, and easy design-in make Hall devices the sensors of choice in hundreds of automotive, aircraft, appliance, and tool applications.
FIG. 3 represents a discrete Hall sensor device 30 consistent with known vertical Hall (VH) technology. As shown, sensor device 30 comprises five contacts 301-305 arranged in a line on top of a deep n-type wafer 310 of about 30 μm. In addition, two P-diffusion wells 320 laterally surround an active area of the Hall sensor device where contacts 301-305 are located. In operation, contact 303 is supplied with a supply voltage Vs and contacts 301 and 305 are grounded so that when a magnetic field Hs, having a direction oriented into the paper is applied, current flows are generated from contact 303 to contacts 302 and 304, and to contacts 301 and 305. Hall voltages VH+ and VH− are thus generated and can be detected at contacts 302 and 304.
FIG. 4 is a schematic diagram showing the distribution of the current flow within the deep n-type substrate 310. As the deep n-type substrate 310 has a depth of about 30 μm, sensor device 30 is open downwards and allows a deep current flow. Since the sensitivity of a Hall sensor decreases as the sensing distance increases, new miniaturization techniques that increases the sensitivity of a Hall sensor device are desirable.
A present trend is to integrate Hall sensors with semiconductor integrated circuits instead of employing discrete Hall sensor ICs. Such integration allows a system approach thereby improving the sensor performance despite the mediocre characteristics of basic Hall cells. Among various integrated circuits, CMOS integrated circuits (Complementary Metal Oxide Semiconductor) are considered preferred over bipolar integrated circuits because CMOS provides a higher level of integration and lower power and cost.
One example of integrating Hall sensors with CMOS is disclosed by E. Schurig et al. in the article entitled “A Vertical Hall Device in CMOS High-Voltage Technology”. The vertical Hall sensor described in this article is built in bulk CMOS, which has a cross-sectional view as shown in FIG. 5.
Similar to the conventional VH sensor of FIG. 3, Hall sensor device 50 of FIG. 5 also comprises five contacts 501-505. These five contacts 501-505, however, are arranged in a line on top of a low-doped, active n-diffusion region 510, which has a depth of about 7 μm. In this known Hall sensor structure, the sensor device 50 also comprises two P-diffusion wells 520 laterally surrounding the active area of the Hall sensor 50.
As the n-diffusion layer 510 in this device has a depth of about 7 μm, the current flow distribution in the sensor can be limited to 7 μm, as shown in a distribution diagram of FIG. 6. With the distribution distance decreased, the concentration of the current flow is closer to the contacts 502 and 504. Thus, the sensitivity of the Hall sensor device to the magnetic field is increased compared to that of the VH sensor device of FIG. 3.
Although Hall sensor device 50 of FIG. 5 has increased sensitivity for detecting a magnetic field in comparison with the VH sensor of FIG. 3, a Hall sensor device having still higher sensitivity is always desired as it helps to simplify overall system designs, reduce cost, and decrease power consumption.